newforms error structure not well documented

I'll try to write up a PR for a documentation change when I get a moment.

It took me a while to figure out why my error dictionaries coming from Django and DRF were not working with newforms (I'd assumed since newforms was inspired by django it might 'just work'). Django and DRF both appear to return errors as:

```
{'fieldName' : [ 'something bad happened', 'other bad thing']}
```

Where as newforms appears to expect errors to be of the form:

```
{'fieldName' : [ {
'code' : 'some',
'message': 'something bad happened',
},
{ 'code' : 'other',
'message': 'other bad thing',
}
]}
```